The measurement of the magnetic characteristics of highly permanent magnetic materials, such as Rare-Earth Transition Metal magnets based on Nd--Fe--B or Sm-Co, conventionally requires the measurement of the hysteresis loop of the magnetic material. The material is first fully magnetised and then taken around its hysteresis loop by demagnetisation of the material. Measurement of the hysteresis loop enables various magnetic parameters of the material to be determined. The three parameters of main importance to permanent magnets are the remanence, the coercivity and the squareness factor. Generally, for permanent magnets, it is considered that high values for each of these parameters are desirable.
The remanence is the value of the magnetisation in the sample after it has be en magnetised, and is measured at zer o applied field. The intrinsic coercivity is the value of the applied demagnetising field required to reduce the magnetisation of an initially magnetised sample to zero. The zero magnetisation condition must be measured while the demagnetising field is being applied. The squareness factor of the magnet relates to the degree with which the magnetisation remains close to its remanence value as a demagnetising field is applied. There are various definitions for the squareness factor, but it always has a value between 0 and 1. Perhaps the most common representation is the ratio of the applied demagnetising field at which the magnetisation falls to 90% of its remanent value, to the intrinsic coercivity.
All three of these values may be obtained from the second quadrant demagnetisation curve of the magnet. To obtain this second quadrant curve, or indeed a full four-quadrant hysteresis loop, three basic type s of system are commercially available namely, permeameter systems, vibrating sample magnetometer systems, and pulsed magnetometer systems.
Permeameter systems employ electromagnets consisting of ferromagnetic iron-based cores and pole pieces, and current-carrying coils, to apply a magnetic field, and multi-turn coils linked to integrating fluxmeters to measure the magnetic properties of a sample in terms of the applied magnetising or demagnetising field and the corresponding magnetisation in the sample. Permeameter systems are accurate, and cheaper than the other two commercially available methods, (vibrating sample magnetometer and pulsed magnetometer, see below), but can only provide magnetic fields of the order of 2MA/m (25kOe), which are not sufficient for use with all highly coercive permanent magnets.
Furthermore, the permeameter method is a closed magnetic loop measurement system, and samples need to be placed and removed from between the pole pieces, which need to be wound open and closed for each sample. This is a time-consuming process, and, coupled with the time taken to obtain the results and calculate the magnetic parameters, makes the permeameter method too slow to be of use in magnetic characteristic production testing systems.
Vibrating sample magnetometer systems use an electromagnet, or, for higher fields, the core of a superconducting solenoid to enable the measurement of the magnetic characteristics. However, superconducting systems require liquefied gasses for their operation, are very expensive and have long run-up times typically in the order of a day which are impractical for many industries. Even the fastest lower-field electromagnet-based systems can take several minutes to measure the relevant magnetic parameters.
Pulsed magnetometer systems require the discharging of large capacitor banks into a resistive solenoid. Pulsed magnetometer systems do provide relatively high strength magnetic fields which are suitable for characterisation of highly permanent magnets. However, both the vibrating sample and pulsed magnetometer systems make open circuit measurements, which are intrinsically less accurate than closed circuit measurements of the permeameter systems. This is because open circuit measurements require demagnetisation calculations and assumptions to be made to estimate the average actual field the sample is subjected to.
Pulsed magnetometer and vibrating sample magnetometer systems work by varying the applied magnetic field to take the magnetic sample around its hysteresis loop. During this procedure, the applied magnetic field and the magnetism of the sample are measured by sampling and are recorded. Once the required data has been obtained and recorded, at the end of the sampling procedure, the recorded data is analysed to determine the required characteristics. This post-measurement analysis of the recorded data is complicated in that open-circuit correction factors are required. In addition, relatively expensive processing power is required and, more importantly, the time taken to obtain the useful values of the magnetic characteristics such as remanence and coercivity is lengthy. The time taken to obtain values of the desired characteristics typically takes minutes and is too long to make these systems economically viable as production testing systems.